Method and apparatus for introduction of high boiling point streams at low temperature

ABSTRACT

The present invention is a method and apparatus for sampling a high-temperature gaseous process stream containing components with high boiling points. The sampling system is especially suited for instruments having extremely low pressure chambers, such as mass spectrometers. The invention reduces the condensation of high boiling point components of the sample in the sampling system without the necessity of maintaining extremely high temperatures. The gaseous sample is passed through an orifice from the high temperature stream into a lower-temperature zone of the sampling system where a low pressure is maintained by a vacuum pump. The low pressure reduces the boiling point of the sample components so they may be maintained in a gas phase without excessive heating. The low pressure sample is then introduced into an instrument chamber through a sample introduction valve.

This is a divisional application of Ser. No. 10/785,304 filed on Feb.24, 2004 and claims the benefit of U.S. Provisional Patent ApplicationSer. No. 60/537,835, filed Jan. 21, 2004 which is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates generally to the field of sampling streamsof gaseous materials for testing, and more particularly, to a method andapparatus for sampling process streams containing high-boiling-pointcomponents, without the need to maintain the sample at excessively hightemperatures.

BACKGROUND OF THE INVENTION

Often when analyzing process streams using mass spectroscopy or otheranalysis techniques, the sampling system is the most important part ofgenerating meaningful data. A properly functioning sampling system mustwithdraw a small amount of material from a process stream, and deliverit to an analysis instrument in a substantially intact condition.

In some process streams, for example acrylonitrile streams, there aresmall amounts of materials that boil at very high temperatures. Thosecomponents of the sample may condense out of the streams and plug thesampling lines. One frequently-used solution to that problem is to heatthe sample stream to extremely high temperatures (180 degrees C.) tokeep those components in the gas phase. That technique requires largeamounts of energy and considerable expense. In some cases, thetemperatures required are so high they cannot be reached using availableequipment. Further, there may be materials present in the stream thatdecompose at such high temperatures, making such a solution impractical.

One example of a mass spectrometry sampling system that is heated toavoid condensation is shown in U.S. Pat. No. 3,944,824 to Chagney et al.That system samples gaseous compounds from a main process line, andincludes a sampling line that is traced with steam to assure that nocondensates reach the mass spectrometer.

U.S. Pat. No. 6,670,608 to Taylor et al. discloses a gas sampling systemfor sampling hazardous process gasses for analysis in an instrument thatis remote from the process line. The technique uses a small-diametercapillary to transport a sample from the process stream to a massspectrometer chamber. The capillary tube is open at one end to the highvacuum environment of the mass spectrometer chamber, and at the otherend to the process stream. The capillary tube diameter is chosen basedon the pressures in the sample stream and the mass spectrometer. Acapillary tube heater is provided to maintain the sample in thecapillary tube above boiling point.

It is known to control the temperature and pressure of a desolvationchamber when preparing a liquid sample for introduction into a massspectrometer chamber. For example, in U.S. Pat. No. 4,403,147 to Meleraet al., a jet stream of liquid droplets is sprayed through a probe intoa low pressure, high temperature chamber for evaporation before enteringthe mass spectroscopy chamber.

There is therefore presently a need to provide a method and apparatusfor sampling a process stream containing high-boiling-point gaseouscomponents for analysis in a test instrument such as a massspectrometer. Particularly, there is a need for a technique that canprevent condensation of the high-boiling-point components in thesampling system without excessively heating the sample. To theinventors' knowledge, no such technique is currently available.

SUMMARY OF THE INVENTION

The present invention addresses the needs described above by providing amethod for sampling a high temperature process stream, withoutpermitting high-boiling-point components to condense in the samplingsystem, and without requiring excessive heating. In one embodiment, themethod includes the steps of evacuating a low temperature zone of asampling system using a vacuum pump, admitting a portion of the hightemperature process stream into the low temperature zone through anorifice, maintaining a stable vacuum pressure in the low temperaturezone, and introducing a sample from the low temperature zone of thesampling system into test equipment through a sample introduction valve.

The orifice may have a diameter of between 0.005 inches and 0.025inches. The step of maintaining a stable vacuum pressure in the lowtemperature zone may include metering an inlet of the vacuum pump, ormay include controlling the vacuum pump.

The temperature of the high temperature process stream may be above aboiling point of a target sample component at the process streampressure. The method may include the step of maintaining a temperatureof the low temperature zone above a boiling point of a target samplecomponent at the stable vacuum pressure.

The test equipment may include a mass spectrometer, and may be a FT-ICRmass spectrometer. The stable vacuum pressure may be a pressure betweena pressure of the process stream and a high vacuum pressure of a vacuumchamber of the test equipment.

In another embodiment of the invention, a sampling system is providedfor sampling a high temperature process stream to be tested in ananalytical instrument. The sampling system includes an evacuation systemfor maintaining a low temperature zone of the sampling system at avacuum pressure, a nozzle having an orifice connecting the sample streamwith the low pressure zone of the sampling system, and a sampleintroduction valve connecting the low temperature zone of the samplingsystem with a vacuum chamber of the analytical instrument. The sampleintroduction valve is located between the evacuation system and thenozzle.

The analytical instrument may be a mass spectrometer and may be a FT-ICRmass spectrometer. The evacuation system may include a vacuum pump, andmay further comprise a metering valve for metering an intake of thevacuum pump.

The orifice may have a diameter of between 0.005 inches and 0.025inches.

Another embodiment of the invention is a method for sampling from agaseous process stream at a process stream temperature and pressure. Thesubject stream has at least one component with a first boiling pointlower than the process stream temperature when at the process streampressure.

The method includes admitting a gas sample from the process streamthrough an orifice into a sampling system. The sampling system has asampling system temperature lower than the first boiling point. Thesampling system further has a sampling system pressure lower than theprocess stream pressure. The component in the gas sample thus has asecond boiling point at the sampling system pressure, the second boilingpoint being lower than the sampling system temperature. The method alsoincludes the step of introducing a portion of the gas sample into a testinstrument chamber.

The step of introducing the portion of the gas sample into the testinstrument chamber may include pulsing a piezoelectric valve. The methodmay also include the step of maintaining a stable vacuum pressure in thesampling system.

The step of maintaining a stable vacuum pressure in the sampling systemmay include regulating a vacuum pump throughput, and may includeregulating a valve that meters flow through a vacuum pump.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the functional elements of asampling system according to one embodiment of the invention.

FIG. 2 is an end view of a nozzle used in accordance with one embodimentof the invention.

FIG. 3 is a sectional view of the nozzle of FIG. 2, taken through lineIII-III.

FIG. 4 is a detail view IV of the nozzle of FIG. 3.

FIG. 5 is a block diagram showing a method of sampling a process streamaccording to one embodiment of the invention.

FIG. 6 is a sample mass spectrum obtained using a method and apparatusaccording to the invention.

FIG. 7 is a reference mass spectrum of the same material analyzed in thespectrum of FIG. 6.

DESCRIPTION OF THE INVENTION

The method and apparatus of the present invention provide a samplingsystem that can be maintained at a relatively low temperature withoutpermitting condensation of the sample. To accomplish that, the inventorshave developed a sampling system that utilizes the well-known principlethat the boiling point of a liquid decreases with the pressure of thesurrounding gasses. By lowering the pressure of the sample stream,problem materials boil at much lower temperatures. Those problemmaterials that tend to condense in the sampling system under ambientpressure will remain in the gaseous phase without the necessity ofapplying excessive heat to the sampling system.

In one example, a material that boils at 336 degrees C. at atmosphericpressure boils at about 140 degrees C. at 1 torr, boils at 90 degrees C.at 50 millitorr and boils at 70 degrees C. at 10 millitorr. Significantgains may therefore be made by lowering the pressure of the samplingsystem.

The sample is delivered to the instrument without substantial condensateand at a pressure lower than ambient pressure. The present invention isparticularly effective in applications involving instruments that sampleat sub-ambient pressures, such as a mass spectrometer.

A sampling system 100 according to the present invention is shownschematically in FIG. 1. The process gas is sampled either directly froma main process line (not shown) or from sampling return lines 108, 109providing gas to the sampling system 100 from the main process line. Inthe exemplary prototype embodiment of the invention described herein,the sampling line 108 and return line 109 are ¼ inch tubing. It shouldbe recognized that while the invention is described herein as aprototype constructed of individual components connected by tubing, anintegral construction or a hybrid construction may be used withoutdeparting from the intended scope of the invention.

The sampling line 108 and return line 109 are in a high temperature zone105 of the sampling system. The high temperature zone 105 is maintainedat a temperature sufficiently high to keep all sample components in agaseous state. The sample passes through the lines 108, 109 at a processpressure that may be atmospheric pressure or slightly above atmosphericpressure, as required by the process at the point where the sample iswithdrawn.

A nozzle 110 having a small orifice is provided in the branch line 108for permitting a small amount of the process gas to enter the lowtemperature zone 120 of the sampling system. A pressure at thedownstream side of the nozzle 110 and throughout the low temperaturezone of the sampling system is maintained by a vacuum pump 150. Thepressure in the low temperature zone is lower than the process streampressure in lines 108, 109. The pressure in the low temperature zone isfurthermore higher than a pressure in the mass spectrometer chamberbeing supplied by the sampling system.

In the prototype system constructed by the inventors, a ¼ inch tube 124was used on the downstream side of the nozzle. That tube was reduced to1/16 inch tubing 125 into the sample introduction valve and 1/16 inchtubing out of that valve. ¼ inch tubing 129 was used in the remainder ofthe low temperature zone 120.

A bypass path 144 is provided in parallel to the sample introductionvalve. The bypass path is constructed of ¼ inch tubing. The bypass path144 reduces restriction, permitting a larger orifice to be used toprevent clogging. The relative diameters of the bypass path (¼ inch) andthe tubing to and from the sample introduction valve ( 1/16 inch) resultin proper flow allocation between the two paths. Alternatively, ametering valve (not shown) may be used in the bypass path.

The vacuum pump 150 is preferably a positive displacement pump such aspiston or vane-type vacuum pump. In an exemplary embodiment of theinvention, the pressure in the low temperature zone is approximately 10torr. One skilled in the art will recognize that other sampling systempressures may be selected based on the pressures of the process streamand the analysis chamber, and on the phase diagram characteristics ofthe gaseous components to be sampled. The vacuum pump preferablyexhausts to an abatement system such as an atmospheric scrubber, as isknown in the art.

An auxiliary heater (not shown) is provided in the low temperature zoneto maintain a temperature in that zone that is sufficient to keep thecomponents of the sample in a gas phase at the reduced pressure.Advantageously, that temperature is considerably lower than thetemperature required to maintain a gas phase of those components atambient pressure.

The pressure in the low temperature zone of the sampling system ismaintained at a stable level by regulating a volumetric rate of thevacuum pump 150 and/or adjusting a flow rate of a metering valve 128placed on the inlet side of the vacuum pump. The metering valve mayalternatively be placed at the exhaust of the vacuum pump. In theprototype embodiment of the invention constructed by the inventors, themetering valve was adjusted manually with reference to a pressure gage122. That adjustment may be automated through the use of a feedbackcontrol system.

An isolation valve 140 and a pressure sensor 142 are installed in-linenear the vacuum pump for start-up and maintenance. A sub-ambientpressure regulator 131 may be installed in the low temperature zone toassist in maintaining a stable pressure at the sampling valve 130

A sample introduction valve 130 is provided for introducing an amount ofthe sample into the analysis chamber. In the case of a Fourier transformion cyclotron resonance (FT-ICR) mass spectrometer, the preferred sampleintroduction valve is a piezoelectric pulse valve. Sample introductionvalves operating on other principles may be use in conjunction withother analysis systems.

In the case of an FT-ICR mass spectrometer, the sample is introducedinto a chamber evacuated to a pressure of about 10⁻¹⁰ torr. While it isnecessary that the pressure in the sampling system be substantiallygreater than that in the MS chamber, it can be seen that there is alarge pressure range between the process stream pressure (approximately1 atmosphere=760 torr) and the MS chamber pressure of 10⁻¹⁰ torr. Apressure in the sampling system may therefore be selected based on thegaseous components present in the sample, and a desired maximum boilingpoint of those components.

A nozzle fitting 200 according to one embodiment of the invention isshown in FIGS. 2, 3 and 4. The nozzle fitting 200 is inserted into amanifold through which the process stream passes.

The nozzle 200 is of generally cylindrical construction, having a wall204 with a thickness of about 1/16 inches and a length 205 of about 1inch. One end of the cylinder is open, while the other has a conicalwall 208 with a cone angle 209 of about 45 degrees.

As best shown in FIG. 4, an orifice 220 is provided at the center of theconical wall 208. The orifice passes through a wall approximately 0.020inches in thickness. In the prototype embodiment of the inventiondiscussed herein, orifices having diameters of 0.010, 0.014 and 0.018inches were tested with good results. In practice, the orifice size 221is selected based on the desired pressure drop between the processstream and the sampling system, and also based on a trial and errordetermination of the effect of the orifice size on clogging and fouling.

A method 500 for sampling a process stream in accordance with theinvention is shown in block diagram form in FIG. 5. The method includesfirst evacuating (step 510) a low temperature zone of the samplingsystem using a vacuum pump. The term “evacuate” as used herein means toreduce a pressure in a vessel to a level below atmospheric pressure, butnot necessarily to remove all contents from the vessel. As noted above,a mechanical, positive displacement vacuum pump such as a piston pump,vane pump or rotary pump is used. The sampling system is evacuatedindependently of the test instrument chamber, using a separate vacuumpump. In the case of an FT-ICR mass spectrometer, high vacuum ismaintained in the test instrument chamber using a molecular pump such asa sputter ion pump. Initial vacuum for the instrument is drawn off-lineusing a mechanical pump.

A portion of the high temperature process stream is then admitted (step520) into the low temperature zone through an orifice. As noted, theorifice diameter is selected based on the relative pressures of thesample system and the process stream, and on the cloggingcharacteristics of the sample. A stable vacuum pressure is maintained(step 530) in the low temperature zone. The pressure may be maintainedby controlling the vacuum pump, a metering valve, or both. Bycontrolling the nozzle size and the amount of pumping, a stable pressureover a very wide range from millitorr to torr values can be reached andmaintained at the sample introduction valve.

A sample is then introduced (step 540) from the low temperature zone ofthe sampling system into a test equipment chamber through the sampleintroduction valve. In the case where the instrument is a FT-ICR massspectrometer, the sample introduction valve is a piezoelectric pulsevalve that can admit extremely small quantities of material at precisepoints in time.

The prototype system discussed above was successfully used by theinventors to analyze dibutylnaphthalene, a high-boiling-point materialthat would likely cause clogging problems in atmospheric pressuresampling systems. The analysis was carried out using a FT-ICR massspectrometer. A resulting mass spectrum from that experiment ispresented in FIG. 6. A reference mass spectrum of the same material isshown in FIG. 7.

The analysis of dibutylnaphthalene was undertaken by placing the purematerial in metal cylinder that was present in the high temperatureregion of a prototype sampling system of the invention. The hightemperature zone 105 (FIG. 1), including the cylinder (not shown), wasmaintained at about 170 degrees C. A flow of nitrogen was passed throughthe cylinder at about 10 ml/minute. The low temperature zone 120 of thesampling system was held at 120 degrees C. The vacuum pump 150 wasswitched on and the metering valve 128 was adjusted until a stablepressure of about 10 torr was attained in the low temperature zone. Thesample introduction valve 130 was then pulsed to admit a portion of thesample gas contained in the low temperature region. The spectrum 600shown in FIG. 6 was generated. It can be seen that that spectrumcorrelates with the reference spectrum 700 shown in FIG. 7. The systemwas run for 30 minutes without any signs of plugging or clogging.

The foregoing Detailed Description is to be understood as being in everyrespect illustrative and exemplary, but not restrictive, and the scopeof the invention disclosed herein is not to be determined from theDescription of the Invention, but rather from the Claims as interpretedaccording to the full breadth permitted by the patent laws. For example,while the sampling method is described primarily for use in connectionwith a FT-ICR mass spectrometer, the technique of the invention may beused as a sampling means for other analysis instruments having a lowpressure chamber into which the sample must be introduced, whileremaining within the scope of the invention. It is to be understood thatthe embodiments shown and described herein are only illustrative of theprinciples of the present invention and that various modifications maybe implemented by those skilled in the art without departing from thescope and spirit of the invention.

1. A sampling system for sampling a high temperature process stream to be tested in an analytical instrument, the sampling system comprising: an evacuation system for maintaining a low temperature zone of the sampling system at a vacuum pressure; a nozzle having an orifice connecting the sample stream with the low pressure zone of the sampling system; and a sample introduction valve connecting the low temperature zone of the sampling system with a vacuum chamber of the analytical instrument, the sample introduction valve being located between the evacuation system and the nozzle.
 2. The sampling system of claim 1, wherein the analytical instrument is a mass spectrometer.
 3. The sampling system of claim 1, wherein the analytical instrument is an FT-ICR mass spectrometer.
 4. The sampling system of claim 1, wherein the evacuation system comprises a vacuum pump.
 5. The sampling system of claim 4, further comprising a metering valve, connected to the vacuum pump.
 6. The sampling system of claim 1, wherein the orifice has a diameter of between 0.005 inches and 0.025 inches.
 7. A sampling system for sampling a high temperature process stream, the sampling system comprising: means for evacuating a low temperature zone in the sampling system; means for admitting a portion of the high temperature process stream into the low temperature zone, the means for admitting connected to the means for evacuating; means for maintaining a stable vacuum pressure in the low temperature zone, the means for maintaining connected to the means for evacuating; and means for introducing a sample from the low temperature zone of the sampling system into test equipment, the means for introducing connected to the means for admitting.
 8. The sampling system of claim 7, wherein the means for admitting comprises a nozzle.
 9. The sampling system of claim 8 wherein the nozzle comprises an orifice having a diameter of between 0.005 inches and 0.025 inches.
 10. The sampling system of claim 7, wherein the means of maintaining a stable vacuum pressure in the low temperature zone comprises a metering valve.
 11. The sampling system of claim 7, wherein the means for maintaining a stable vacuum pressure in the low temperature zone comprises a pressure regulator.
 12. The sampling system of claim 7, wherein the means for evacuating comprises a vacuum pump.
 13. The sampling system of claim 7, further comprising a heater connected to the means for admitting for maintaining a temperature of the low temperature zone above a boiling point of a target sample component at the stable vacuum pressure.
 14. The sampling system of claim 7, further comprising a mass spectrometer connected to the means for introducing a sample.
 15. The sampling system of claim 7, further comprising a FT-ICR mass spectrometer connected to the means for introducing a sample.
 16. The sampling system of claim 7, wherein the means for evacuating comprises a first vacuum pump and wherein the test equipment comprises a mass spectrometer and further comprising a second vacuum pump, the second vacuum pump connected to a chamber of the mass spectrometer for evacuating the chamber to a pressure lower than the stable vacuum pressure in the low temperature zone. 